What do you call a female nightingale?

Answer: hen

A female nightingale is called a hen. The Nightingale (Luscinia megarhynchos), also known as Rufous and Common Nightingale, is a small passerine bird that was formerly classed as a member of the thrush family Turdidae, but is now more generally considered to be an Old World flycatcher, Muscicapidae. It belongs to a group of more terrestrial species, often called chats.